Mobile communication techniques have become a widely used communication manner for their advantages such as flexibility and convenience since 1980s. In a lot of mobile communication standards, CDMA cellular communication technique shows great potential for its features associated with large capacity, simple frequency planning, good communication quality and small electromagnetic interference. IS-95 CDMA cellular communication system proposed by Qualcomm Inc. and rapidly developed all over the world uses this CDMA cellular communication technique. Several candidate schemes of the third generation of digital cellular communication system are established on the basis of CDMA techniques.
Multipath fading which causes serious multipath interference exists in a mobile communication system. In general, it is necessary to receive pilot signals with confirmation information so as to evaluate the amplitude and phase information of multipath signals, and it is possible to achieve multipath diversity and coherent reception. A coherent spread spectrum receiver which performs diversity process is referred to as RAKE coherent receiver. RAKE coherent receiver can correct phases of a plurality of singlepath signals which carry same information and are independence from one another in fading features, and perform maximal combination to overcome multipath fading and improve received signal-to-interference ratio.
To achieve RAKE reception function, Synchronizing local spread spectrum sequence (PN code) with received signal is necessary. The synchronization is achieved by acquiring and tracking steps. The acquiring step acquires a pilot channel and confirms that initial synchronization (coars synchronization) of PN code is complete. The combination of these two steps provides PN code and accurate local timing required for RAKE receiver.
Mobile communication spread spectrum receivers in CDMA cellular communication system have capabilities of diversity combination receipt to achieve soft handoff so as to improve receiving performance at boundaries of cells.
CDMA receivers have large local oscillation frequency-offset in turn-on losing lock state due to the limit of cost. An automatic frequency correction (AFC) function is introduced into the receivers so that the RAKE receivers can operate normally in large local oscillation frequency-offset state.